In the field of driver assistance functions having forward-looking detection systems, radar sensor systems are used, which operate primarily in the frequency range of 76 GHz to 77 GHz. These are used, for example, for implementing the “adaptive cruise control” (ACC) assistance function in the speed range of 50 km/h to 180 km/h. Radar sensors are also used for applications in the lower speed range and are advantageous for other comfort and safety functions such as blind spot monitoring, backing and parking assistance, or “pre-crash functions” (deployment of reversible restraint systems, arming of airbags, etc., preconditioning of the brake system, automatic emergency brake).
Typically, 77 GHz radar sensors operate using lens antennas. A plurality of partially overlapping beam lobes is formed over a plurality of feed antennas which are located in the focal plane of the lens (“analog” beam formation). FIG. 1 illustrates this principle. The azimuthal angle position of the target object is determined using the signal amplitudes and/or signal phases in the individual beam lobes. The relatively high overall depth of a few centimeters resulting from the required distance of the feed antennas (in the focal plane) from the lens is characteristic for lens antennas.
“Analog” beam formation may, however, also be achieved with a planar structure using planar antennas, so that the overall depth is substantially reduced. Corresponding circuits for beam formation such as the Butler matrix, Blass matrix, or planar lenses (Rotman lens) are known (German Patent No. DE 199 51 123). A planar group antenna is used as the antenna.
However, other methods for signal analysis, in particular for determining radar target angles, which require no “analog” beam formation, are also known. The received signals are processed and digitized separately for each of the antenna elements, and the beam is formed on the digital level (“digital” beam formation). In addition to the “digital” beam formation, there are also methods using which the azimuthal angle position of the target object may be determined without any need for beam formation, e.g., high-resolution direction estimation methods.
A particularly simple and cost-effective design of a planar antenna is based on serial feed of the elements in one dimension of the antenna. Serial feed in the antenna columns is relevant in particular for motor vehicle radar sensors. In this case, the columns are situated in the elevation direction, i.e., vertically.
Slot couplings in connection with patch elements are known per se (U.S. Patent Publication No. 2003 010 75 18, PCT Patent Publication No. WO-2002 07 1535, European Patent Application No. EP 1199772). Such slots are used for adapting and influencing the bandwidth.